Illumination system utilizing two opposing DC lamps with an optimum barrel-shaped slit

ABSTRACT

.[.A symmetrical light intensity distribution profile is achieved by placing a pair of similar asymmetrical linear DC gas discharge lamps in parallel position but opposed in polarity. The individual asymmetrical distributions of the lamps combine in a symmetrical total intensity distribution. A desired &#34;butterfly&#34; or other intensity distribution profile can be achieved..]. .Iadd.The invention relates to an illumination system which provides nonuniform illumination along an object plane in the dimension of an illumination source which includes two linear lamps. The lamps have their anodes and cathodes connected in reverse fashion relative to each other; each lamp provides its own asymmetrical light output distribution pattern but the resultant output of both lamps is a symmetrical pattern which provides the characteristic nonuniform illumination on the object plane. .Iaddend.

.Iadd.This application is a reissue of U.S. Pat. No. 4,086,010, grantedon Apr. 25, 1978. .Iaddend.

BACKGROUND

This invention relates generally to illumination and more particularlyto an illumination system for use in a photocopying environment for thepurpose of tailoring the illumination profile of an object or documentto be copied.

In one form of photocopying apparatus in which this invention might beapplied, a flat document platen supports an original document forimaging through a projection lens disposed along the optical pathbetween the object platen and a photoreceptive surface disposed at theimage plane of the lens. The photoreceptor is typically disposed on arotating cylindrical drum. Because of the curvature of the photoreceptordrum, a narrow exposure slit is disposed adjacent to the drum and alongits length to limit the image field incident on the photoreceptor.

It is desirable to provide uniform exposure across the width of thephotoreceptor, that is along the length of the exposure slit. In orderto accomplish this, the object illumination must be nonuniform. Onereason for this, as is well known, is that light propagating from anobject to its image attenuates outwardly from the image center inproportion to the cos⁴ θ, where θ is the angle subtended by any spot inthe image plane with the optical axis of the system. Another reason whynonuniform document illumination is desirable is the typical use of anexposure slit having other than uniform width along its length. A morethorough discussion of this latter consideration is to be found inapplication Ser. No. 737,301, .Iadd.now abandoned .Iaddend.filedherewith by Simpson and Rees, the disclosure of which is herebyincorporated by reference in the present specification.

It is an object of this invention to provide an illumination system bywhich an object is nonuniformly illuminated, thus to provide uniformexposure of the object at its image plane.

SUMMARY OF THE INVENTION

This invention is practiced in one form by a pair of similar linear DCgas discharge lamps mounted side-by-side but opposed in polarity. Suchlamps inherently produce an asymmetrical light intensity distribution.By the side-by-side and polarity-reversed mounting, a desired"butterfly" or other intensity distribution profile can be achieved.

For a better understanding of this invention, reference is made to thefollowing more detailed description given in connection with theaccompanying drawing.

DRAWINGS

FIG. 1 is an optical diagram of an imaging system showing oneenvironment for application of this invention.

FIG. 1a is a modification of part of the system shown in FIG. 1.

FIG. 2 represents a linear gas discharge lamp.

FIG. 3 is an idealized light intensity distribution profilecorresponding to the lamp of FIG. 2.

FIG. 4 represents a linear gas discharge lamp similar to that in FIG. 2but reversed in polarity.

FIG. 5 is an idealized light intensity distribution profilecorresponding to the lamp of FIG. 4.

FIG. 6 represents the lamps of FIGS. 2 and 4 placed side-by-side.

FIG. 7 is an idealized light intensity distribution profile of thecombination shown in FIG. 6.

FIG. 8 represents the lamps of FIG. 6 in a slightly modifiedarrangement.

DESCRIPTION

Referring to FIG. 1, an optical system is generally indicated at 2 andincludes an object platen 4 disposed in an object plane 6, and aphotoreceptor surface 8 disposed at an image plane 10. Photoreceptor 8may be mounted for rotation on a cylindrical drum 12 which is tangent tothe image plane 8. A projection lens 14 is disposed on an optical axis16 between the object and image planes. A diaphragm member 18 defines anaperture or exposure slit 20 therethrough. A pair of light sources 22and 26 is located under the object platen 4 to illuminate an objectthereon for projection of its image onto the rotating photoreceptor 8.Means represented by arrow 24 are provided to effect scanning movement,either of the object relative to stationary optics or of the opticsrelative to a stationary object.

FIG. 1a shows the same environment as FIG. 1 except that lamps 22 and 26are placed side-by-side in combination as a unit, one such unit on eachside of the optical axis 16. Lamps 22 and 26 are identical DC lampsexcept that they are reversed in polarity relative to each other.

Referring to FIG. 2, a DC gas discharge lamp is represented at 30 andincludes an anode electrode 32 and a cathode electrode 34, theseelectrodes being suitably connected to a DC power source (not shown). Alamp envelope 36 extends between electrodes 32 and 34 and contains theactive gas or vapor. Low pressure mercury argon fluorescent lamps are anexample of a vapor lamp that may be used according to this invention.

When the circuit connected to the lamp of FIG. 2 is closed, currentflows through the lamp as indicated by the arrow, and the light emittedfrom the lamp has an intensity distribution profile represented in FIG.3. As is observed, the lamp is brighter at the cathode end than at theanode end. FIG. 3 is illustrative and not definitive of the intensityprofile, which may or may not be linear.

FIG. 4 shows a similar lamp 40 oppositely oriented with its cathode 44at the left and its anode 42 at the right, and an accompanying lightintensity distribution profile in FIG. 5 which is just the opposite ofthat shown in FIG. 3.

FIG. 6 shows the lamps 30 and 40 positioned in combination to emit alight intensity distribution profile represented in FIG. 7. FIG. 7 canbe considered substantially a summation of the curves of FIGS. 3 and 5and shows the light intensity greater at the ends of the lampcombination than at the center.

FIG. 8 shows two opposed DC lamps that are slightly offset to providefurther control of the light distribution. From this it will beappreciated that the profile of FIG. 7 can be "compressed" or"stretched" by altering the relative axial positions of the lamps.

There are several factors which influence the light intensitydistribution from a DC gas discharge lamp. These include current, coldspot orientation with respect to lamp polarity, and gas or vaporpressure within the lamp envelope as discussed in patent application,Ser. No. 595,936, filed July 14, 1975, by Gallo and Hammond .Iadd.andissued as U.S. Pat. No. 4,005,332 on Jan. 25, 1977 .Iaddend., thedisclosure of which is incorporated herein by reference.

With respect to current, higher currents through a DC gas discharge lamptend to accentuate the asymmetry of intensity distribution representedin FIGS. 3 and 5. Accordingly, higher currents will tend to increase theintensity contrast between the ends and the center of the FIG. 7profile.

As to cold spot orientation, the asymmetry of FIGS. 3 and 5 is greater.[.than.]. .Iadd.when .Iaddend.the cold spot is located at the cathodeend of the lamp. Again, this variable can be used to accentuate theintensity profile of FIG. 7.

Gas or vapor pressure is also a factor here, the asymmetry of FIGS. 3and 5 increasing as the lamp vapor pressure decreases. This again isvariable to control the intensity profile of FIG. 7.

It will be apparent that this invention permits a control over thedistribution of light output intensity along the length of a gasdischarge lamp. This control capability finds one practical applicationin tailoring the illumination profile of an object which is to beoptically imaged. Cos⁴ θ exposure attenuation at the image plane, andother factors such as exposure slit non-uniformity, can be compensatedfor to provide constant exposure across the breadth of thephotoreceptor.

The foregoing description of an embodiment of this invention is given byway of illustration and not of limitation. The concept and scope of theinvention are limited only by the following claims and equivalentsthereof which may occur to others skilled in the art.

What is claimed is:
 1. An optical system for projecting an image from anobject plane to an image plane, including the following disposed alongan optical axis:a. an object platen to support an object in said objectplane, b. projection lens, c. a photoreceptor disposed in said imageplane, and d. an illumination .[.system.]. .Iadd.source .Iaddend.toilluminate said object platen .[.including.]. .Iadd.said sourcecomprising: .Iaddend.1. a first linear lamp disposed adjacent to andparallel to said object platen, said first lamp being a DC gas dischargelamp having an anode at one end and a cathode at the other end thereofand having .[.an.]. .Iadd.a first .Iaddend.asymmetrical distributionpattern of output light intensity along its length,
 2. a second linearlamp similar to the first and disposed parallel to said first lamp, withthe anode and cathode of said second lamp disposed at the ends thereofin the reverse polarity with respect to said first lamp, .[.the.]..Iadd.said second lamp having a second asymmetrical distribution patternof light intensity opposite to that of said first asymmetricaldistribution pattern, said first and second .Iaddend.asymmetricaldistribution patterns of light output intensity .[.of said first andsecond lamps.]. combining in a resultant symmetrical distributionpattern of output light intensity to illuminate said object platen withnon-uniform, symmetrical light intensity along the dimension of saidplaten corresponding to the length of said lamps to compensate foroff-axis light attenuation at said image plane.
 2. An optical system asdefined in claim 1 in which said first and second linear lamps aredisposed adjacent to each other forming an integral light source.
 3. Anoptical system as defined in claim 2, further including a second suchintegral light source disposed parallel to the first and on the oppositeside of said optical axis from said first integral light source.
 4. Anoptical system as defined in claim 1, and further including:means toeffect scanning of said object platen by said illumination .[.system.]..Iadd.source .Iaddend.to incrementally illuminate .Iadd.portions of.Iaddend.the object thereon, .[.a diaphragm defining an exposure slitadjacent to said photoreceptor to expose only a narrow strip of saidphotoreceptor to said image,.]. whereby .[.a composite image is.]..Iadd.said illuminated portions are .Iaddend.incrementally projected onsaid photoreceptor .Iadd.by said lens.Iaddend..
 5. An optical system asdefined in claim 4 in which said resultant symmetrical distributionpattern of output light intensity is of a generally butterflyconfiguration corresponding to greater light intensity at the ends thanat the center thereof. .[.6. A method of creating a symmetricaldistribution pattern of light output intensity in an extended lightsource including:placing a first linear DC gas discharge lamp and asecond linear DC gas discharge lamp in parallel juxtaposition with theirpolarities opposed so that inherent asymmetrical distribution pattern ofoutput light intensity along said first lamp is substantially equal andopposed to the inherent asymmetrical distribution pattern of outputlight intensity along said second lamp, adjustably moving said lampsaxially relative to each other to vary the symmetrical distributionpattern of light output intensity as desired..]. .Iadd.
 7. In aphotocopier, an optical system for illuminating a document to be copiedat an object plane and for forming an image of said document at animaging plane including an integral illumination source comprising thefollowing disposed along an optical axis (a) a first linear lampdisposed adjacent to and parallel to said object plane said first lampbeing a DC gas discharge lamp having an anode at one end and a cathodeat the other end thereof and having a first asymmetrical distributionpattern of output light intensity along its length, (b) a second linearlamp similar to the first and disposed parallel to said first lamp, withthe anode and cathode of said second lamp disposed at the ends thereofin the reverse polarity with respect to said first lamp, said secondlamp having a second asymmetrical distribution pattern of lightintensity opposite to that of said first asymmetrical distributionpattern, said first and second asymmetrical distribution patterns oflight output intensity combining in a resultant symmetrical distributionpattern of output light intensity to illuminate said object plane withnon-uniform symmetrical light intensity along the dimension of saidobject plane corresponding to the length of said lamps, and means forforming an image of said document at said imaging plane. .Iaddend..Iadd.8. An optical system as defined in claim 7 in which said first andsecond linear lamps are disposed adjacent to each other forming anintegral light source..Iaddend. .Iadd.9. An optical system as defined inclaim 8, further including a second such integral light source disposedparallel to the first and on the opposite side of said optical axis fromsaid first integral light source. .Iaddend. .Iadd.10. An optical systemfor transmitting an image from an object in an object plane to an imageplane, including the following disposed along an optical axis:(a) anillumination source to illuminate said object plane including:
 1. afirst linear lamp disposed adjacent to and parallel to said object planesaid first lamp being a DC gas discharge lamp having an anode at one endand a cathode at the other end thereof and having an asymmetricaldistribution pattern of output light intensity along its length,
 2. asecond linear lamp similar to the first and disposed parallel to saidfirst lamp, with the anode and cathode of said second lamp disposed atthe ends thereof in the reverse polarity with respect to said firstlamp, the asymmetrical distribution patterns of light output intensityof said first and second lamps combining in a resultant symmetricaldistribution pattern of output light intensity to illuminate said objectplane with a non-uniform symmetrical light intensity along the dimensionof said platen corresponding to the length of said lamps, and (b) lighttransmitting means adapted to image said illuminated object onto saidimage plane. .Iaddend. .Iadd.11. An optical system for transmitting animage of an object in an object plane onto a photosensitive image planeincluding the following disposed along an optical axis: an illuminationsource to illuminate said object including a first linear lamp disposedadjacent to and parallel to said object plane and on one side of theoptical axis, said first lamp being a DC gas discharge lamp having ananode at one end and a cathode at the other end thereof and having afirst asymmetrical distribution pattern of output light intensity alongits length, a second linear lamp disposed in the same position relativeto the object plane as said first lamp but on the opposite side of saidoptical axis, said second lamp being a DC gas discharge lamp with itsanode and cathode connected at the ends thereof in reverse polarity withrespect to said first lamp, said second lamp having a secondasymmetrical distribution pattern of output light intensity along itslength, said first and second lamps having a resultant symmetricaldistribution of light output intensity at said object plane which isgreater at the ends of the lamp combination then at the center, means toeffect scanning of said object plane by providing relative motionbetween said object plane and said illumination source therebyincrementally illuminating the object on said object plane with saidresultant light pattern, and light transmitting means adapted to imagesaid illuminated object onto said photosensitive image plane. .Iaddend..Iadd.12. An optical system as defined in claim 1 further includingmeans to move said first and second lamps axially relative to each otherwhereby said non-uniform symmetrical light intensity along the platendimension is varied. .Iaddend. .Iadd.13. An optical system as defined inclaim 1 wherein said linear lamps are low pressure fluorescent lamps..Iaddend. .Iadd.14. A photocopier as defined in claim 7 wherein saidlinear lamps are low pressure fluorescent lamps. .Iaddend. .Iadd.15. Anoptical system as defined in claim 11 wherein said linear lamps are lowpressure fluorescent lamps. .Iaddend.